Late Silurian zircon U–Pb ages from the Ludlow and Downton bone beds, Welsh Basin, UK

Abstract

The Ludlow Bone Bed (Welsh Basin) is a critical stratigraphic horizon and contains a rich assemblage of fish scales. Units above provide insights into the early evolution of animal and plant life. The bed has not yet been radioisotopically dated. Here, we report 207 secondary ion mass spectrometry (SIMS) ages from 102 zircon (ZrSiO4) grains from the Ludlow (n = 2) and stratigraphically higher Downton (n = 1) bone beds. SIMS ages are middle Ordovician (471.6 ± 20.7 Ma) to late Devonian (375.7 ± 14.6 Ma, 238U–206Pb, ±1σ analytical uncertainty). Cathodoluminescence images show that the youngest ages appear affected by alteration. Chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb geochronology was utilized to improve precision. Detrital zircon grains from Downton yield 424.91 ± 0.34/0.42/0.63 Ma and from Ludlow 424.85 ± 0.32/0.41/0.62 Ma (n = 5 each, 238U–206Pb, ±2σ analytical, tracer or systematic uncertainty). These ages provide a maximum deposition age. Results overlap the basal Přídolí age (423.0 ± 2.3 Ma) in its stratotype (Požáry Section, Reporyje, Prague, Czech Republic). The Ludlow Bone Bed marks the base of the local Downton Group, which has previously been correlated with the base of the Přídolí Series. The CA-ID-TIMS ages are older than those for other land arthropod-bearing sediments, such as the Cowie Harbour Fish Bed and Rhynie Chert.Supplementary material: An Excel file containing detailed information on the SIMS analyses, a figure showing calibration curves for AS3 standards sputtered over sessions 1 and 2, and a figure showing CA-ID-TIMS U–Pb age data (concordia and weighted mean plots) are available at https://doi.org/10.6084/m9.figshare.c.5087031

How old is the Ordovician–Silurian boundary at Dob’s Linn, Scotland? Integrating LA-ICP-MS and CA-ID-TIMS U-Pb zircon dates

“…Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Secondary Ion Mass Spectrometry (SIMS) allow for rapid U-Pb dating of zircons to determine provenance and maximum depositional ages (MDAs) (Table 1). Some studies show that LA-ICP-MS and SIMS methods have systematic biases in 238 U- 206 Pb zircon dates relative to those obtained using the more precise yet destructive Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry method (CA-ID-TIMS) (Mattinson, 2005;Allen & Campbell, 2012;Crowley et al 2014;Marillo-Sialer et al 2014;Von Quadt et al 2014;Watts et al, 2016;Catlos et al 2021). For these reasons, understanding the U-Pb zircon system and potential systematic biases is crucial to produce accurate dates for MDAs of sections of interest.…”

“…Furthermore, many local biostratigraphic schemes for different areas cannot be accurately correlated between marine and non-marine sections. Thus, the North American, British and Scandinavian schemes suffer from a number of correlation problems, and the Mediterranean and North Gondwanan schemes, and it is complicated to relate to the standard Series and Stages (Sweet & Bergström, Thermal Ionization Mass Spectrometry method (CA-ID-TIMS) (Mattinson, 2005;Allen & Campbell, 2012;Crowley et al 2014;Marillo-Sialer et al 2014;Von Quadt et al 2014;Watts et al, 2016;Catlos et al 2021). For these reasons, understanding the U-Pb zircon system and potential systematic biases is crucial to produce accurate dates for MDAs of sections of interest.…”

Chemical Diversity of Teeth and Bone Fragments from a Newly Discovered Upper Muschelkalk Bone Bed from Silesia, Poland

“…Bone beds are unusually rich accumulations of mostly apatitic bones, scales and teeth from more than one individual organism that occurs in the same geological stratum. They vary in age and are hosted by sediments with different lithologies (i.e., Devonian, Silurian, Permian, Cretaceous) (e.g., [4][5][6][7]).…”